Boiler tube protected against corrosive substances



Jan. 22, 1952 J, R H 2,583,015

BOILER TUBE PROTECTED AGAINST CORROSIVE SUBSTANCES Filed April 7, 1945 Watented Jan. 22, 1952 Roman TUBE PROTECTED AGAINST CORROSIVE SUBSTANCES John Alexander Reich, Chester, Va., assignor to Combustion Engineering Superheater, Inc., New York, N. Y., a corporation of Delaware Application April 7, 1945, Serial No. 587,051

This invention relates to boiler furnaces of the type in which the furnace is defined in'part or in whole by tubular water walls or other similar water walled surfaces and in which the type of firing is such as tocause slagging on the tube or portions of the tubes.

The nature, objects and advantages of the invention will be clear from the following:

The invention will be described in connection with a moderntubular water wall lined furnace, in which a very large proportion of the steam is generated by the water walls and which is fired with pulverized coal and is equipped with a socalled slagging bottom. Installations of this type are-constructed to operate at very high pressures, say, for-example, 1400 to 2000 p. s. i. and for very large steaming capacity.

Considerable difficulty has been encountered in such high pressure, high capacity installations with respect to the water wall tubes, due to rapid erosion or corrosion. Slag forms on the water wall tubes from adjacent to the bottom up toward the upper portion of the combustion chamber. In some instances, particularly in large furnaces,

the tubes will havethe deposit extend as. high as twenty feet from tlie'bottorn. In'others, it may go only as high as six to eight feet. The extent of the tubes, longitudinally considered, which may have slag formed thereon, will, of course, also vary with the rate of firing. At extremely low rates, for example, the tubes may have slag thereon, say only a few feet, and much farther up as the rate of firing increases. The nature of the slag also varies somewhat, depending upon the type of fuel and the rate of firing. At very low rates of firing the slag may build up to a substantial thickness and may be somewhat porous in nature next to the tubes due to the cooling effect of the tubes. When the deposit has built up to a point sufiicient to be substantially unaffected by the cooling action of the tubes, the surface of the deposit will become molten and some of it will run down to the bottom of the furnace. At higher rates of firing the deposit will be of less thickness and substantially all of it will be molten, in which condition more of it runs to the bottom of the furnace. From time to time, slag falls off the tubes. On sharp changes in rate of firing this occurs very extensively and, in effect, the tubes are deslagged.

It has long been known that the rapid erosion of tubes above referred to is related with the slagging conditions, the generally accepted theory being that the running down of the slag, particularly at the high rates of firing, physically :r

caused the erosion.

An investigation of the causes, disclosed that the injury to the tubes by erosion or corrosion is due to the action of one or more of the chemical constituents of the slagging and introduced 2 Claims. (Cl. 122-235) into the furnace with the coal. It has also been found that the generally enamel-like deposits which appear on the tubes beneath slag in the areas where the corrosion occurs, consist primarily of a solid solution of sodium and potassium sulphates and alkali metal ferric trlsulfates. such as NaIiFe(SO4)3 and K3Fe(SO4)3. Experiments with these enamel-like deposits have shown that at temperatures normally occurring in the tubes of high pressure steam generators, sodium and potassium sulphates will react read- 11y with the iron oxide of the tubes in an atmosphere containing sulphur trioxide even in low concentrations, to form the same alkali metal sulfates as occur in the deposits. However, under the same conditions of temperature and concentration of sulfur trioxide, neither the iron oxide alone nor the alkali metal sulphates alone will react with the sulfur trioxide. These reactions suggest that the mechanism of corrosion involves the removal of the normal protective oxide coating on the furnace tubes by (a) the condensation on the relatively cool tubes of alkali metal oxides which are converted to the corresponding sulphates by sulfur trioxide in the furnace, and (b) the subsequent reaction of the iron oxide on the tubes and the alkali metal sulphates with sulfur trioxide, evolved as a result of the slagging reactions in the coal ash deposited on the enamellike deposits. Thus conditions are afforded for the removal of the iron oxide on the tube with the forming of alkali metal ferric trisulphates. Deslagging causes thermal decomposition of these compounds with the removal of the iron oxide from the tubes, and the cycle is repeated.

I have found that when tubes which are subject to corrosion under the conditions described above, are coated with metallic aluminum, the corrosion substantially ceases. Tests have shown that under conditions under which said alkali metal sulphates and sulfur trioxide react with the normal oxide on low carbon steel to form alkali ferric 'trisulphates, metallic aluminum will remain substantially unaffected. Photomicrographs of tubes coated with metallic aluminum which were removed, after having been in use in a furnace for a substantial period of time, have shown that the coating remains substantially unaffected and will continue to afford protection to the tube against the corrosion above described.

Whether or not the theor es above set forth are correct, the fact remains that if the tubes be provided with a metallic aluminum coating over the regions of the tubes which would normally be subject to slagging conditions, corrosion ceases. The essence of the invention, therefore, is the provision of such a coating for the tubes, at least over the regions in which the tubes are subjected to slagging conditions, in a furnace in which the type of firing produces such slagging conditions.

It is obvious that the invention is applicable to any type of firing in which such slag is produced. For example, in the high pressure, high capacity stoker fired installation, there are distinct localized regions in which slagging on the tubes ocours.

The coating of metallic aluminum may be applied in ways well understood in this art, as, for example, by metal spraying with or dipping in metallic aluminum. The requisite is that the metallic aluminum coating shall be adherently bonded to the tube. While I have described certain specific alkali metal sulphates and a specific oxide of sulfur as entering into the reactions above described, it may well be that other metallic sulfates and oxides of sulfur may react similarly with the iron oxide.

In'the accompanying drawing 1 have shown a vertical section through a water wall lined tubular furnace, fired by pulverized coal and having a slagging bottom from which the molten slag is tapped. This type of furnace is shown and described, for example, in United States Patent to Jackson et al., No. 2,069,834, issued February 9, 1937.

In practicing the invention, the tubes comprising the water walls will be covered with the metallic aluminum coating described, at least over the regions in which slag is deposited in detrimental quantities on such tubes. If desired, also. the bottom tubes and one or more of the first rows of tubes in advance of the superheater, may be so coated.

I claim:

1. In a boiler furnace, steam generating carbon steel tubes of conventional formulation exposed to the products of combustion, said furnace having a type of firing producing ski-881 8 on said tubes and burning fuel producing sulfur oxides and alkali oxides in the products of combustion. said tubes being provided with a firmly bonded 4 metallic aluminum coating in the regions where slags ns occurs on saidtubes and where the temperature is sufilciently low to bring about condensation of alkali oxides in the gases which oxides are converted into alkali sulfates in the presence of said sulfur oxides.

2. In a boiler furnace, steam generating carbon steel tubes of conventional formulation, said tubes being exposed to the products of combustlon and constituting wall and slag collecting bottom portions of the furnace. said furnace havin a type of firing producing slagging on said tubes and burning fuel producing sulfur oxides and alkali oxides in the products of combustion, said tubes being provided with a firmly bonded metallic aluminum coating in the regions where slagging occurs on said tubes and where the temperature is sufficiently low to bring about condensation of alkali oxides in the gases which oxides are converted into alkali sulfides in the presence of said sulfur oxides.

JOHN ALEXANDER REICH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 678,615 Forg July 16, 1901 1,904,218 Forward Apr. 18, 1933 FOREIGN PATENTS Number Country Date 699,059 France Dec. 8, 1930 OTHER REFERENCES Pages 8 and 9 of the publication entitled, Diamond Soot Blowers," published March 1936, by Diamond Power Specialty Corporation, of Detroit. Michigan. 

1. IN A BOILER FURNACE, STEAM GENERATING CARBON STEEL TUBES OF CONVENTIONAL FORMULATION EXPOSED TO THE PRODUCTS OF COMBUSTION, SAID FURNACE HAVING A TYPE OF FIRING PRODUCING SLAGGING ON SAID TUBES AND BURNING FUEL PRODUCING SULFUR OXIDES AND ALKALI OXIDES IN THE PRODUCTS OF COMBUSITON, SAID TUBES BEING PROVIDED WITH A FIRMLY BONDED METALLIC ALUMINUM COATING IN THE REGIONS WHERE SLAGGING OCCURS ON SAID TUBES AND WHERE THE TEMPERATURE IS SUFFICIENTLY LOW TO BRING ABOUT CONDENSATION OF ALKALI OXIDES IN THE GASES WHICH OXIDES ARE CONVERTED INTO ALKALI SULFATES IN THE PRESENCE OF SAID SULFUR OXIDES. 